The present invention relates generally to the field of fossil fuel cyclone-fired boilers and, in particular, to the reduction of NO.sub.x compounds produced during the combustion of such fossil fuels in the boiler.
The Clean Air Act Amendments of 1990 have created significant challenges for electric power generation utilities and other industry producers to substantially reduce both SO.sub.x and NO.sub.x emissions. The Act mandates reduction of NO.sub.x from stationary sources. The sections of the Act dealing with acid rain require utilities to use low-NO.sub.x burner technology. Other sections require the use of reasonable, available control technology to reduce NO.sub.x emissions from both utility and industry sources. The resulting impact is that by January 2000, in excess of 200,000 MW.sub.e of electricity generating capacity must be retrofitted with low-NO.sub.x systems.
The limitations imposed by the Act are particularly challenging to achieve with cyclone-fired boilers. A cyclone furnace generally consists of a cyclone burner connected to a horizontal water-cooled cylinder called the cyclone barrel. Air and crushed coal are introduced through the cyclone burner into the cyclone barrel. Larger coal particles are thrust out to the barrel walls by a cyclonic motion of combustion air where they are captured and burned in the molten slag layer that forms on the barrel walls. Smaller particles burn in suspension. The mineral matter melts and exits the cyclone via a tap at the cyclone throat which leads to a water-filed slag tank. Combustion gases and remaining ash exit the cyclone and enter the main furnace.
Currently, cyclone-fired boilers account for approximately 26,000 MW.sub.e of generating capacity in the United States, or approximately 15% of pre-New Source Performance Standards (NSPS) coal-fired generating capacity. These units contribute approximately 21% of NO.sub.x emissions produced by pre-NSPS coal-fired units.
Typical low-NO.sub.x burners and staged combustion techniques do not work in cyclones because these techniques rely on the creation of an oxygen deficient, or reducing, atmosphere to hamper the formation of NO.sub.x compounds. Creating a reducing atmosphere within a cyclone firing typical high sulfur, high iron fuels is not practical due to the corrosion of tubes which would occur and the resulting maintenance costs and problems. Cyclones firing these fuels must operate with excess oxygen in the cyclone barrel, and this condition coupled with high temperatures and severe turbulence within the cyclone barrel are the reasons why cyclone-fired boilers are disproportionately high sources of NO.sub.x emissions.
Reburn technology offers cyclone-fired boiler operators an alternative to expensive flue gas cleanup techniques for reducing NO.sub.x emissions by injecting supplemental fuel, such as oil, coal, natural gas (or other) into the main furnace to create locally reducing conditions which convert NO.sub.x produced in the main combustion zone to molecular nitrogen, thereby reducing the total amount of NO.sub.x emissions.
Several modifications to cyclones have been used to attempt to reduce NO.sub.x emissions from these boilers. Conference papers discussing these alternatives include: G. J. Maringo, et al., "Feasibility of Reburning for Cyclone Boiler NO.sub.x Control", EPA/EPRI Joint Symposium on Stationary Combustion NO.sub.x Control, New Orleans, La., Mar. 23-27, 1987; H. Farzan, et al., "Pilot Evaluation of Reburning for Cyclone Boiler NO.sub.x Control", EPA/EPRI Joint Symposium on Stationary Combustion NO.sub.x Control, San Francisco, Calif., Mar. 6-9, 1989; H. Farzan, et al., "Reburning Scale-Up Methodology for NO.sub.x Control from Cyclone Boilers", International Joint Power Generation Conference, San Diego, Calif., Oct. 6-10, 1991; A. S. Yagiela, et al., "Update On Coal Reburning Technology for Reducing NO.sub.x in Cyclone Boilers", EPA/EPRI Joint Symposium on Stationary Combustion NO.sub.x Control, Washington, DC, Mar. 25-28, 1991; H. Farzan, et al. "Evaluation of Reburning NO.sub.x Control and Coal Switching in a Cyclone Boiler", Engineering Foundation Conference on Coal Blending and Switching of Western Low Sulfur Coals, Snow Bird, Utah, Sept. 26-Oct. 1, 1993; H. Farzan, et al., "Gas Reburn Retrofit on an Industrial Cyclone Boiler", EPA/EPRI Joint Symposium on Stationary Combustion NO.sub.x Control, Kansas City, Mo., May 16-19, 1995; H. Farzan, et al., "NO.sub.x Reduction Using Natural Gas Reburn on an Industrial Cyclone Boiler", International Joint Power Generation Conference, Houston, Tex., Oct. 14-16, 1996. These papers generally discuss the problems of NO.sub.x emission reductions and some solutions are offered.
Examples of known solutions described in some of the articles for reducing NO.sub.x emissions from cyclone boilers are also shown in U.S. Pat. Nos. 5,052,312 and 5,022,329. In these patents, a cyclone furnace for burning hazardous wastes is disclosed which creates a reducing condition inside the cyclone furnace adjacent the main furnace. A burner is used to inject fuel adjacent to the end of the cyclone barrel. The fuel is directed outwardly by the burner nozzles and immediately mixes with the swirling air and fuel present in the cyclone.
A cyclone after-burner for reducing NO.sub.x is disclosed in U.S. Pat. No. 5,572,956 to Hallstrom et al. and has a retractable fuel pipe within a lance which extends through the cyclone to the re-entrant throat of the main furnace. Reburn fuel is not provided within the cyclone, but rather to a point beyond the cyclone.